The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Layered heaters are typically used in applications where space is limited, when heat output needs vary across a surface, where rapid thermal response is desirous, or in ultra-clean applications where moisture or other contaminants can migrate into conventional heaters. A layered heater generally comprises layers of different materials, namely, a dielectric and a resistive material, which are applied to a substrate. The dielectric material is applied first to the substrate and provides electrical isolation between the substrate and the electrically-live resistive material and also reduces current leakage to ground during operation. The resistive material is applied to the dielectric material in a predetermined pattern and provides a resistive heater circuit. The layered heater also includes leads that connect the resistive heater circuit to an electrical power source, which is typically cycled by a temperature controller. The lead-to-resistive circuit interface is also typically protected both mechanically and electrically from extraneous contact by providing strain relief and electrical isolation through a protective layer. Accordingly, layered heaters are highly customizable for a variety of heating applications.
Layered heaters may be “thick” film, “thin” film, or “thermally sprayed,” among others, wherein the primary difference between these types of layered heaters is the method in which the layers are formed. For example, the layers for thick film heaters are typically formed using processes such as screen printing, decal application, or film dispensing heads, among others. The layers for thin film heaters are typically formed using deposition processes such as ion plating, sputtering, chemical vapor deposition (CVD), and physical vapor deposition (PVD), among others. Yet another series of processes distinct from thin and thick film techniques are those known as thermal spraying processes, which may include by way of example flame spraying, plasma spraying, wire arc spraying, and HVOF (High Velocity Oxygen Fuel), among others.
The resistive heating layer in these layered heaters is generally formed as a pattern or a trace with curved or bend portions, e.g. non-linear, where current crowding often occurs. Generally, current crowding refers to a non-uniform distribution of current density where the current tends to build up or increase near geometric features that present obstacles to a smooth current flow, i.e. bend portions. In operation, as the current travels around a bend portion, the current exhibits a tendency to build up, or crowd, around the inner portion of the curve as it makes its way around the bend portion. Due to this current crowding effect, the bend portions are susceptible to an increased current density, causing burning, which can lead to premature failure of the resistive heating layer and thus the overall heater system.